The present invention relates to clutches and, more particularly, to a bi-directional electro-mechanical and electro-hydraulic overrunning clutch for providing four wheel drive capability with automatic backdrive.
In recent years there has been a tremendous demand for off-road and all terrain vehicles. The interest in these types of vehicles has led to a wide variety of innovations. Many of the innovations have centered around making the vehicle more adaptable to changing road conditions, e.g., dirt roads, pavement and gravel. As the road terrain changes, it is desirable to vary the driving capabilities of the vehicle to more efficiently navigate the new terrain. Prior four-wheel drive and all terrain vehicles were cumbersome since they required the operator to manually engage and disengage the secondary drive shaft, e.g., by stopping the vehicle to physically lock/unlock the wheel hubs. Improvements in vehicle drive trains, such as the development of automated systems for engaging and disengaging a driven axle, eliminated many of the problems of the prior designs. These automated drive systems are sometimes referred to as xe2x80x9con-the-flyxe2x80x9d four wheel drive. Many of these systems, however, require the vehicle to be in either 2-wheel or 4-wheel drive at all times.
Generally, all four-wheel drive vehicles include a differential for transferring torque from a drive shaft to the driven shafts that are attached to the wheels. Typically, the driven shafts (or half shafts) are independent of one another allowing differential action to occur when one wheel attempts to rotate at a different speed than the other, for example when the vehicle turns. The differential action also eliminates tire scrubbing, reduces transmission loads and reduces understeering during cornering (the tendency to go straight in a corner). There are four main types of conventional differentials: open, limited slip, locking, and center differentials. An open differential allows differential action between the half shafts but, when one wheel loses traction, all available torque is transferred to the wheel without traction resulting in the vehicle stopping.
A limited slip differential overcomes the problems with the open differential by transferring all torque to the wheel that is not slipping. Some of the more expensive limited slip differentials use sensors and hydraulic pressure to actuate clutch packs locking the two half shafts together. The benefits of these hydraulic (or viscous) units are often overshadowed by their cost, since they require expensive fluids and complex pumping systems. The heat generated in these systems, especially when used for prolonged periods of time, may also require the addition of an auxiliary fluid cooling source.
The third type of differential is a locking differential that uses clutches to lock the two half shafts together or incorporates a mechanical link connecting the two shafts. In these types of differentials, both wheels can transmit torque regardless of traction. The primary drawback to these types of differentials is that the two half shafts are no longer independent of each other. As such, the half shafts are either locked or unlocked to one another. This can result in problems during turning where the outside wheel tries to rotate faster than the inside wheel. Since the half shafts are locked together, one wheel must scrub. Another problem that occurs in locking differentials is twichiness when cornering due to the inability of the two shafts to turn at different speeds.
The final type of differential is a center differential. These types of differentials are used in the transfer case of a four wheel drive vehicle to develop a torque split between the front and rear drive shafts.
Many differentials on the market today use some form of an overrunning clutch to transmit torque when needed to a driven shaft. One successful use of an overrunning clutch in an all terrain vehicle is disclosed in U.S. Pat. No. 5,036,939. In that patent, the vehicle incorporates overrunning clutches where the wheel hub mounts to the axle, thus allowing each wheel to independently disengage when required.
Another successful use of an overrunning clutch in a differential is disclosed in U.S. Pat. No. 5,971,123, commonly owned by the assignee of the present invention. That patent describes an innovative electro-mechanical bi-directional overrunning clutch differential which addressed many of the problems inherent in the prior drive systems. The bi-directional overrunning clutch differential utilized electrically controlled coils to advance and retard a roll cage, thereby controlling the ability of the differential to engage and disengage depending on the operational state of the primary and secondary wheels. The bi-directional differential in U.S. Pat. No. 5,971,123 also describes a backdriving system. The backdriving system operates by controlling the energizing of selected coils to actively engage the secondary shafts in certain situations where extra traction is needed. For example, when the vehicle is driving down a slope the system engages the front wheels, which are the wheels with the better traction.
The backdrive mechanism in the bi-directional differential disclosed in U.S. Pat. No. 5,971,123, like the overrunning clutch mechanism, uses coils to drag and advance the roll cage for engaging and disengaging the shafts.
One of the drawbacks in the prior overrunning clutch designs is that tolerances between the output hubs and the clutch housing have to be closely controlled since any variation effects when the rolls would engage. Poor tolerance control between an output hub and a clutch housing could result in less than all the rolls engaging at the same time, reducing the amount of torque that can be transferred. Also, in these prior art designs, if the slots in the roll cage are not properly located so that all the rolls are in the exact same position with respect to the wedging surfaces of the clutch housing and hubs, all the rolls may not engage at the same time.
Furthermore, in bi-directional overrunning clutches with two independent output hubs, it has been determined that the tolerances or spacing between the two output hubs and the clutch housing must be the same so that both output hubs engage at the same time. Any significant variation between the two can result in one output hub engaging while the other does not.
A need, therefore, exists for an improved spring assembly for a roll cage that permits uniform roll engagement to accommodate variations in tolerances or spacings between rolls in a roll cage and a clutch housing. A need also exists for an improved bi-directional overrunning clutch which includes improved spring assemblies located between each output hub and the clutch housing for providing uniform and simultaneous engagement of both output hubs.
The present invention relates to a spring assembly for biasing rolls in a bi-directional overrunning clutch. The clutch includes a roll cage disposed between a clutch housing and at least one race. The roll cage includes a plurality of rolls located within slots formed in the roll cage, the rolls being movable with respect to the roll cage.
In one embodiment, the spring assembly includes a bridge having two ends. A first spring attached to one end of the bridge. The first spring has arms extending in opposite directions from the bridge. Each arm has an end adapted to contact a roll.
A second spring is attached to the other end of the bridge. The second spring has arms extending in opposite directions from the bridge. Each arm has an end adapted to contact a second roll.
The spring assembly is adapted to bias two adjacent rolls of the roll cage in opposite directions from each other.
In another embodiment, a bi-directional overrunning clutch is disclosed with two sets of spring assemblies, one set located between an associated output hub and a clutch housing.
The foregoing and other features and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments thereof, as illustrated in the accompanying figures.